Synthesis of fulleropyrrolidine derivatives and their applications in inverted perovskite solar cells
[Objective]Inverted perovskite solar cells(PSCs)have attracted great attention due to their advantages,including large-scale low-temperature processing,improved operational stability and suitability for tandem cells.However,the most commonly used fullerene electron transport layer(ETL)[6,6]-phenyl-C61-butyric acid methyl ester(PCBM)has high manufacturing cost due to its complex synthesis.Thus,developing low-cost,solution-processable fullerene electron transport materials is crucial for the commercialization of inverted PSCs.Herein,two fulleropyrrolidine derivatives,F1 and F2,were synthesized as alternative low-cost ETL to replace PCBM in inverted PSCs.[Methods]Inverted p-i-n perovskite solar cells were fabricated with the structure of ITO/poly(triaryl amine)(PTAA)/perovskite/fullerene/bathocuproine(BCP)/Ag.PTAA(2 mg/mL in toluene)was spin-coated onto a pre-cleaned ITO substrate at 2 000 r/min for 30 s and annealed at 100 ℃ for 10 min.The perovskite precursor solution was prepared by dissolving a mixed powder of formamidinium iodide(FAI),methylammonium iodide(MAI),CsI,PbI2,and PbBr2 in N,N-dimethylformamide/dimethyl sulfoxide(4:1,by vol)solutions.The precursor solution was spin-coated at 1 000 r/min for 15 s and then 5 000 r/min for 30 s,followed by chlorobenzene antisolvent treatment before the end of the spin-coating process.PCBM,F1 and F2(20 mg/mL in chlorobenzene)were spin-coated at 3 000 r/min for 20 s.Then,BCP(0.5 mg/mL in isopropanol)was spin-coated at 6 000 r/min for 30 s and annealed at 80 ℃ for 10 min.Finally,about 100 nm of Ag was thermally evaporated under high vacuum(2X10-4 Pa)to complete the device.[Results]F1 and F2 were synthesized using a one-pot Prato reaction.which is simple and cost-efficient.UV-Vis spectrometry and cyclic voltammetry measurements show that F1,F2 and PCBM have nearly the same HOMO and LUMO energy levels.PSCs based on F2 exhibit a power conversion efficiency(PCE)of 19.86％,which is much higher than that of F1-based devices(18.62％)and is very close to that of the PCBM-based control devices.Electron mobility tests show that F2,with a benzoate side chain,has higher electron mobility than that of F1 with an alkyl ester side chain,resulting in a higher fill factor(FF)for F2-based devices.Moreover,F2-based devices also exhibit better device stability,retaining 80％of their initial PCEs after 2 500 h of storage in a nitrogen-filled glovebox,while F1-based and PCBM-based devices can only maintain 74％of initial PCEs.[Conclusion]Two fulleropyrrolidine derivatives F1 and F2 were designed and synthesized using Prato reaction,which were used as ETLs in inverted PSCs.The energy levels of these two fullerene derivatives were investigated by UV-Vis spectrometry and cyclic voltammetry,and their photovoltaic performances were also evaluated.The results show that F2,featuring a benzoate side chain,has higher electron mobility than F1 with an alkyl ester side chain,leading to improved FF and PCE in F2-based PSCs.As a result,F2-based devices achieved a PCE of 19.86％,which is very close to that of PCBM-based control devices fabricated under the same conditions.The results indicate that different side chains on fulleropyrrolidine derivatives could have a great impact on their photovoltaic performances.This work provides valuable insights for the development of low-cost,solution-processable fullerene ETLs for highly efficient and stable inverted PSCs.
perovskite solar cellfulleropyrrolidineelectron transport layerphotovoltaic performance
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